The invention relates to a method of operating a variable-speed submersible pump. More particularly, the invention relates to operating such a pump with an electronic speed control unit, which may be mounted immediately adjacent to the pump motor, inside the submersible pump unit.
Submersible electric pumps have been known for many years. One common form of such pump consists of an electric motor attached to a centrifugal pump, the whole assembly being arranged to be lowered to the bottom of a well or borehole and connected to the surface by a cable for the supply of electric power and a hosepipe for the delivery of water. Such pumps are typically controlled by switching on and off the power supply, either automatically or manually, in response to demand for water at the surface.
Pumps for emptying drainage sumps and the like are also known, which are controlled by level switches. Such pumps typically start automatically when a level switch detects that the sump is full, and stop automatically when another level switch indicates that the sump is empty.
With both of those types of pump, there is typically no attempt to regulate the speed or output of the pump: it is simply either running or stopped. That means that the motor and pump are seldom running at their most efficient states, and that extremely conservative cut-off limits have to be set to prevent overloading or overheating of the motor or pump, requiring unduly large motors and pumps.
Frequency-controlled variable speed motors are known. However, when using such motors in submersible applications, it is usual to mount the controller in a dry situation, which necessitates a long power lead carrying the variable-frequency supply from the controller to the motor. That can result in serious electromagnetic interference (EMI) problems.
There is therefore a need for a submersible electric pump the speed of which can be optimized accurately to the exact operating conditions of the particular site at the particular time, which does not cause EMI problems, and which is economical, compact, and easy to install.
According to one aspect of the present invention, there is provided a method of operating a variable-speed pump driven by a motor capable of developing high torque at low speeds. The method comprises attempting to run the pump at a selected speed. It is determined, by assessing the torque being generated by the motor, whether the pump is clogged. When the torque being developed by the motor exceeds a maximum for the selected speed, the speed of the pump is reduced and the maximum acceptable torque for the motor is thereby increased. The torque is then permitted to rise unless and until it exceeds a maximum for the newly selected speed. The steps of reducing the speed and permitting the torque to rise are repeated, but if at any point it is determined that the pump is no longer clogged, normal pumping is resumed.
If the selected speed is reduced to a minimum speed and it is determined that the pump is still clogged, the pump may be run backwards, and the steps of running the pump forwards to try to clear the clog may be repeated. The pump may be run backwards a second time, forwards a third time, and backwards a third time.
The pump may be run forwards at maximum speed for a predetermined period of time, in a further attempt to clear the clog.
In another aspect of the invention, there is provided a method of operating a pump for emptying a tank that may contain both liquid and solids. The method comprises determining whether the tank is sufficiently full for emptying to be appropriate. If emptying is appropriate, the pump is run backwards to agitate and aerate the tank. After agitating and aerating the tank, the pump is run forwards to remove contents from the tank. If emptying is not appropriate, whenever a predetermined period has elapsed since the pump was last run, the pump is run backwards to agitate the tank.
The tank may be determined to be sufficiently full if the level of liquid in the tank is higher than a certain level.
The pump may be stopped when it is determined that the tank is empty. It may be determined that the tank is empty when the torque consumed by the pump decreases suddenly.
When emptying is not appropriate, the pump may be run backwards at a minimum operating speed of the pump. When the pump comprises an electric motor, the temperature of the motor may be monitored, and the pump may be stopped if its temperature is too high. The motor may then be restarted after a predetermined time, or when its temperature falls to a predetermined value.
An alarm may be emitted if the level of liquid in the tank exceeds an alarm level higher than the level at which emptying of the tank is appropriate.
According to another aspect of the invention, there is provided a method of operating a variable-speed pump, comprising running the pump at a selected speed. The pumping rate and pump speed are monitored, and the speed is incremented until either the pump is pumping fluid at a predetermined rate, or the pump is operating at a maximum speed.